A variety of memory types are employed in semiconductor devices and integrated circuits. One type of memory gaining in use is ferroelectric memory, which is a non-volatile memory in that stored data is maintained without power and/or frequent refresh cycles.
A ferroelectric memory cell employs a ferroelectric capacitor and, for example, a (metal oxide semiconductor) MOS transistor. The construction can be similar to the storage cell of a DRAM. However, the ferroelectric capacitor employs a ferroelectric material as a dielectric layer between top and bottom electrodes. This ferroelectric material has a high dielectric constant and can be polarized by an electric field. The polarization remains until an opposite electrical field reverses it. This makes the memory non-volatile.
Ferroelectric materials are characterized by a reversible polarization in the absence of an electric field. The polarization in a ferroelectric material arises from a noncentrosymmetric arrangement of ions in its unit cell that produces an electric dipole moment. Adjacent unit cells tends to polarize in the same direction and form a region called a ferroelectric domain.
In principle, the operation of ferroelectric memory devices is based on the hysteretic behavior of polarization with electric field. When an external voltage is applied to a ferroelectric capacitor, there is a net ionic displacement in the unit cells of the ferroelectric material. The individual unit cells interact constructively with their neighbors to produce domains within the material. As voltage is removed, the majority of the domains will remain poled in the direction of applied field, requiring compensation charge to remain on the plates of the capacitor. It is this compensation charge that causes the hysterisis in the polarization with applied external voltage. For example, at zero applied field, there are two states of polarization. Either of these two states could be encoded as a “1” or a “0” and since no external field is required to maintain these states, the memory device is nonvolatile. To switch the state of the device, an applied voltage greater than a threshold amount is required. The direction of the applied voltage results in one of the two states of polarization.